Method and apparatus for collecting photons in a solid state imaging sensor

ABSTRACT

A photon collector has a reflecting metal layer to increase photon collection efficiency in a solid state imaging sensor. The reflecting metal layer reflects incident light internally to a photosensor. A plurality of photon collectors is formed in a wafer substrate over an array of photosensors. The photon collector is formed in an opening in an insulating layer provided over each photosensor.

FIELD OF THE INVENTION

The present invention relates to solid state imaging sensors, and moreparticularly to metal coated photon collectors used in solid stateimaging sensors.

BACKGROUND OF THE INVENTION

Solid state imagers generate electrical signals in response to lightreflected by an object being imaged. Complementary metal oxidesemiconductor (CMOS) imaging sensors are one of several different knowntypes of semiconductor-based imagers, which include for example, chargecoupled devices (CCDs), photodiode arrays, charge injection devices andhybrid focal plane arrays.

Some inherent limitations in CCD technology have promoted an increasinginterest in CMOS imagers for possible use as low cost imaging devices. Afully compatible CMOS sensor technology enabling a higher level ofintegration of an image array with associated processing circuits wouldbe beneficial to many digital image capture applications. CMOS imagershave a number of desirable features, including for example low voltageoperation and low power consumption. CMOS imagers are also compatiblewith integrated on-chip electronics (control logic and timing, imageprocessing, and signal conditioning such as A/D conversion). CMOSimagers allow random access to the image data, and have lowermanufacturing costs, as compared with conventional CCDs, since standardCMOS processing techniques can be used to fabricate CMOS imagers.Additionally, CMOS imagers have low power consumption because only onerow of pixels needs to be active at any time during readout and there isno charge transfer (and associated switching) from pixel to pixel duringimage acquisition. On-chip integration of electronics is particularlydesirable because of the potential to perform many signal conditioningfunctions in the digital domain (versus analog signal processing) aswell as to achieve reductions in system size and cost.

Nevertheless, demands for enhanced resolution of CCD, CMOS and othersolid state imaging devices, and a higher level of integration of imagearrays with associated processing circuitry, are accompanied by a needto improve the light sensing characteristics of the pixels of theimaging arrays. For example, it would be beneficial to minimize if noteliminate the loss of light transmitted to individual pixels duringimage acquisition and the amount of crosstalk between pixels caused bylight being scattered or shifted from one pixel to a neighboring pixel.

Accordingly, there is a need and desire for an improved solid stateimaging device, capable of receiving and propagating light with minimalloss of light transmission to a photosensor. There is also a need anddesire for improved fabrication methods for imaging devices that providea high level of light transmission to the photosensor and reduce thelight scattering drawbacks of the prior art, such as crosstalk.

SUMMARY OF THE INVENTION

Exemplary embodiments of the invention include an imaging device, animage pixel array in an imaging device, and an imager system havingimproved characteristics for reception and transmission of light byphotosensors. Enhanced reception of light is achieved by fabricatingphoton collectors having highly reflecting internal surfaces. The photoncollectors of the invention are formed as individual units operable tocollect and reflect light to corresponding individual photosensors.

Also provided are methods for forming an imaging device, in accordancewith exemplary embodiments of the invention, including forming photoncollectors disposed over focal plane arrays of photosensors. Theexemplary embodiments provide methods of fabricating an imaging devicethat include forming a plurality of photosensors on a wafer, disposingan insulating material over the photosensors, etching openings in theinsulating material sufficient to allow light to reach the photosensors,and coating the inner sidewalls of the openings with a highly reflectinglayer. Additional steps include filling the openings with an opticallytransparent material, such as spin-on glass, and providing an opticallytransparent etching layer to protect the photosensor. A color filterlayer is also fabricated with an individual color filter over arespective photosensor and a microlens structure layer is fabricatedover the color filter layer.

These and other features and advantages of the invention will be moreapparent from the following detailed description that is provided inconnection with the accompanying drawings illustrating exemplaryembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a portion of a conventional imaging deviceillustrating light transmission paths;

FIG. 2 is a diagram of a portion of an imaging device exemplifying anembodiment of the invention;

FIG. 3 depicts a cross-sectional view of a portion of an imaging deviceand the initial formation of a pixel array in accordance with oneembodiment of the invention;

FIG. 4 depicts a cross-sectional view of the FIG. 3 device at thebeginning of the formation of a photon collector in accordance with theinvention;

FIG. 5 depicts a cross-sectional view of the FIG. 3 device formed at astage of processing subsequent to that shown in FIG. 4;

FIG. 6 depicts a diagram of a fabrication step at a stage of processingshown in FIG. 5;

FIG. 7 depicts a cross-sectional view of the FIG. 3 device formed at astage of processing subsequent to that shown in FIG. 5;

FIG. 8 depicts a cross-sectional view of the FIG. 5 device formed at afinal stage of processing;

FIG. 9 depicts a schematic cross-sectional view of an alternative photoncollector structure of the invention; and

FIG. 10 illustrates a diagram of a processor-based system incorporatingan imaging device fabricated according to the present invention, andwherein the imaging device contains one or more pixel arrays formed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific embodiments which exemplify the invention. These embodimentsare described with sufficient detail to enable those skilled in the artto practice the invention, and it is to be understood that otherembodiments may be employed, and that structural and logical changes maybe made without departing from the spirit or scope of the presentinvention.

The terms “substrate” and “wafer” can be used interchangeably in thefollowing description and may include any semiconductor-based structure.The structure should be understood to include silicon, silicon-oninsulator (SOI), silicon-on-sapphire (SOS), doped and undopedsemiconductors, epitaxial layers of silicon supported by a basesemiconductor foundation, and other semiconductor structures. Thesemiconductor need not be silicon-based. The semiconductor could besilicon-germanium, germanium, or gallium arsenide. When reference ismade to the substrate in the following description, previous processsteps may have been utilized to form regions or junctions in or over thebase semiconductor or foundation.

Referring to the drawings, where like reference numbers designate likeelements, the structure and function of an exemplary imager 20 accordingto the invention, shown in FIG. 2, will be described in relation to aprior art imager 10 depicted in FIG. 1. For the sake of simplicity, theinvention will be described in the environment of a CMOS imager;however, it should be understood that the invention may be implementedwith a CCD imager and other solid state imagers as well. In addition,exemplary embodiments of the invention are described as using photodiodephotosensors; however, it should be understood that other photosensorsmay also be used.

The illustrated portion of the conventional imager 10 includesphotodiodes 11 and 12 as the photosensors formed in a substrate 14,above which a lens structure 16 is supported. Incident light rays 17 arefocused by lens structure 16 onto respective photodiodes 11, 12.Off-axis light rays 18, 19, for example, pass through the lens structure16, however, and impinge respectively on neighboring pixel photodiodes12 and 11, generating “crosstalk” such that light that should have beendetected by one pixel is actually detected by another pixel.

FIG. 2 illustrates a first embodiment of the invention in which aportion of the imager 20 has photodiodes 21 and 22 formed in substrate24. Photon collectors 25, 26 provided above the photodiodes 21, 22 havesidewalls featuring interior reflecting layers 27. Incident light 28impinges either directly on photodiodes 21, 22, or indirectly afterbeing reflected internally by inner layers 27. Reflecting layers 27 areformed of a highly reflecting material, such as aluminum or silver, toprovide substantially total reflection. Consequently, all, or nearlyall, of the light entering the photon collectors 25, 26 is directedrespectively to photodiodes 21, 22. Little, if any, of the off-axislight impinges on neighboring pixels. As a result, crosstalk is reduced,and photon collection efficiency is increased.

An exemplary method of fabricating a CMOS imaging device to includeinterior reflecting layers 27 is described with reference to FIGS. 3-8.FIG. 3 depicts a semiconductor substrate 110 on which a plurality ofphotosensors, e.g., photodiodes 112, has been fabricated. The substrate110 includes integrated circuits and other semiconductor componentsincorporated conventionally in a CMOS chip device that have been omittedfrom the drawings for clarity. The photodiodes 112 are fabricated usingconventional techniques and are shown to illustrate one environment ofmany in which the present invention may be employed.

An etch stop layer 114 is formed over the top of the photodiodes 112prior to formation of an insulating layer 116 using processes andmaterials known in the art. Layer 116 may be formed of BPSG, TEOS, orother optically transparent materials and may be the insulator employedas an interlayer dielectric (ILD) between conductive traces of an imagerarray. Etch stop layer 114 is formed of an optically transparent etchstop material and remains in place over the photodiodes 112. Suitableoptically transparent etch stop materials include silicon nitride(Si₃N₄), for example.

Photodiodes 112 are depicted in a symmetric arrangement and orientation,with an intervening space separating each respective photodiode 112 insubstrate 110. The image pixel array of the invention is preferablyformed with minimal space between each adjacent photodiode. It should beunderstood, however, that the invention is applicable to otherphotosensor arrangements and orientations, to be integrated compatiblywith other components of the device.

As depicted in FIG. 4, openings 124 are formed in insulating layer 116over each photodiode 112. The openings 124 are formed by etching down toetch stop layer 114 using processes known in the art. Alternatively, noetch stop layer 114 is provided, and etching is stopped prior todamaging the surface of the photodiode with the etching process, whileachieving sufficient depth to provide a opening that, once reflectivelycoated, will prevent crosstalk between pixels. Although FIG. 4 depictsetched openings 124 as having substantially diagonal sidewall surfaces128, it must be noted that the etched openings 124 are not limited inshape or dimensions to the depiction in the accompanying drawings, butinstead can be formed to any shape and dimensions desired consistentwith the light-reflecting properties described below. The etchedopenings 124 can, for example, have a cylindrical or conical shape, orcan be formed of combined concave and cylindrical shapes, as describedbelow with reference to FIG. 9. In addition, the sidewalls 128 can haveconcave or convex-curved surfaces. The openings typically will featureupper portions having a larger diameter than at the base of theopenings, and shoulders 130 formed between each opening.

FIG. 5 illustrates a reflecting layer 134 formed on sidewalls 128 andshoulders 130 in accordance with one aspect of the invention. Afteretched openings 124 are formed, as depicted in FIG. 4, opticallyreflecting material is deposited within each opening 124 to formreflecting layer 134 on sidewalls 128 and shoulders 130. Depositing thereflecting material on the shoulders 130 also blocks light from goingthrough the shoulders, and thus further limits crosstalk. The reflectinglayer 134 may include any material that is optically reflecting,preferably about 100% optically reflecting. Exemplary materials includemetallic silver (Ag) and aluminum (Al). Ag is less preferred as comparedto Al because a barrier layer is required beneath the Ag layer toprevent Ag ion diffusion into insulating layer 116. Suitable barrierlayer materials for use with an Ag layer are known in the art andinclude titanium nitride (TiN) and titanium tungsten (TiW).

FIG. 6 is a schematic diagram of an exemplary apparatus and method fordepositing reflecting layer 134 on sidewalls 128 and shoulders 130 ofILD layer 116 using collimated angle sputter deposition. Reflectinglayer 134 is deposited using a sputter target 144 directed at anoff-axis angle θ through a collimator 146. The angle θ is chosen toprevent deposition of reflecting material on etch stop layer 114 overphotodiodes 112. The substrate 110 is rotated during deposition(indicated by arrow A) to achieve better uniformity in the depositedlayer 134. The layer 134 is deposited to a thickness sufficient toprovide total or substantially total reflection. A thickness of 1000 Åis typically enough to do this and prevent visible light transmissionthrough layer 134.

Referring to FIG. 7, an optically transparent layer 154 is provided inthe openings 124 over reflecting layer 134. In an exemplary embodiment,layer 154 is a spin-on dielectric (SOD), which forms a smooth uppersurface and requires no chemical mechanical polishing (CMP).Alternatively, layer 154 can be an optically-transparent flowable oxideor photoresist. Examples include pure silica SODs such as Spinfil™ 400series, and most preferably Spinfil™ 450, and photoresists such asMFR-401LL, available from JSR Corporation. Other transparent materialscan also be used, with CMP if needed.

As illustrated in FIG. 8, a color resist layer or color filter array(CFA) 164, containing different color filters, is formed over eachphotodiode 112, schematically represented as being directly applied totransparent filler 154 and insulating layer 116. Resist layer 164 isprovided such that a single color filter is formed over each photodiode112. Various filtering schemes are known in the art, including additivearrangements of red, green, and blue (RGB) filters, and subtractivearrangements of cyan, magenta and yellow (CMY). In an RGB filter system,for example, since white light includes red, green, and blue light, anexemplary color resist layer 164 includes red, green, and blue filterspositioned over respective photodiodes 112. Any combination or array ofcolor filters, including RGB filters arranged in a so-called Bayerpattern, can be formed for color processing and imaging by an imagingdevice formed in accordance with the present invention.

An optional lens structure 166 is disposed above the filter resist 164and serves to focus light toward the photodiodes 112. Other layers canbe provided. For example, a separate nitride liner (not shown) can alsobe formed along the upper surfaces 126 of each opening 124 to guardagainst diffusion of impurities into the wafer or substrate 110 from thecolor filter materials comprising the color resist layer 164.

FIG. 9 is a schematic diagram of a photon collector 172 according toanother embodiment of the invention, in which a compound opening 174 isformed over photodiode 112 and serves as a photon collection device.Opening 174 has both a cylindrical portion 176 and a straight conicalportion 178. The sidewalls of conical opening 174 are lined with amaterial such as Al or Ag providing substantial or complete totalreflection and the opening is filled with an SOD, for example. Otherconfigurations are possible based on design constrains and intendedapplication. Over the opening 174 are a CFA and spin-on glass/resistlayer 180, and a lens structure 182.

One particular advantage of the invention is that it provides theability to reduce the size of the imager pixel structure. An imagerpixel structure can be described according to the following equationwhich relates to the diffraction limited spot radius of the focusedlight directed to the photodiode:$R = \frac{1.22 \cdot \lambda \cdot L}{n \cdot D}$

In the equation, R is the diffraction limited spot radius, which definesthe point at which light will be focused on the photodiode. A iswavelength, L is the imager stack thickness (FIG. 9), n is the index ofrefraction, and D is the micro-lens aperture (FIG. 9). The values for λand n typically are fixed for any given imager. The general industrytrend is for D to get continually smaller as pixel sizes become furtherminiaturized. As pixel sizes get smaller, R must reduce concomitantly.The only remaining variable available to reduce the diffraction limitedspot size R is the thickness L.

According to the invention, the size of the pixel can be reducedsignificantly because light can be focused higher in the photoncollector, above the photodiode, effectively reducing the L (thickness)term in the spot radius equation above, as compared to the prior artstructure having no photon collector. Consequently, the spot diameter,and thus the pixel size, can be reduced.

The concern in the prior art was to focus most of the light entering thephoton collector to impinge directly on the photodiode, thereby reducingcrosstalk between pixels. The photon collector of the present inventioneliminates that concern, and allows the light to be focused higher,above the photodiode. In the prior art, focusing the light above thephotodiode would have directed light to adjacent pixels, resulting incrosstalk. Due to the total reflection of the photon collector optics ofthe invention, however, light focused higher, closer to the lens 182 atthe top of the opening 174 in FIG. 9, for example, will neverthelessimpinge on the intended photodiode due to reflection or diffraction, andwill not scatter or be shifted to neighboring photodiodes. Consequently,the effective L (thickness) term in the spot size equation above can bereduced significantly because the light can be focused at the top of thehole, closer to the lens. For example, current CMOS imagers having athickness L equal to or about 5.0 μm can be reduced to have an effectivethickness to about 1.1 μm, or substantially the thickness of CFA andspin-on glass/resist layer 180 of FIG. 9.

FIG. 10 illustrates a processor-based system 1100 including an imagingdevice 1108. The imaging device 1108 includes an imager having aphotosensor array, with the photocollection structures as described inconnection with FIGS. 2-9. The processor-based system 1100 is exemplaryof a system having digital circuits that could include image sensordevices according to the invention. Without being limiting, such asystem could include a computer system, camera system, scanner, machinevision, vehicle imaging and navigation, video phone, surveillancesystem, auto focus system, and other systems and devices.

The processor-based system 1100, for example a camera system, generallycomprises a central processing unit (CPU) 1102, such as amicroprocessor, that communicates with an input/output (I/O) device 1106over a bus 1104. Imaging device 1108 also communicates with the CPU 1102over the bus 1104, and includes an imager including photon collectorsaccording to the invention and associated image forming circuits, suchas array processing circuits which produce an image signal. Theprocessor-based system 1100 also includes random access memory (RAM)1110, and can include removable memory 1115, such as flash memory, whichalso communicates with CPU 1102 over the bus 1104. Imaging device 1108may include an image processor, such as a CPU, digital signal processor,or microprocessor, with associated memory storage on a single integratedcircuit or with an image processor on a different chip from thatcontacting the image array.

While the invention is preferably directed to methods for forming photoncollectors for use in CMOS imaging arrays and devices, and the resultingstructures, one skilled in the art will recognize that the invention canbe used to form any type of photon collector array for integration withone or more processing components in a semiconductor device.

The present invention offers an imager structure with high lightcollecting efficiency that can be mass-produced at a reasonable cost.The instant methods produce a CMOS imager having minimal crosstalk.Since a lens structure can be eliminated, the present invention obviatesconcerns about lens radius limitations and about damaging lenses duringdie attach, backgrind, and mounting processes.

It should again be noted that although the invention has been describedwith specific reference to CMOS imaging devices comprising a structurefor transmitting light to a photodiode, the invention has broaderapplicability and may be used in any semiconductor imaging apparatus.Similarly, the fabrication process described above is but one method ofmany that may be used. The above description and drawings illustrateexemplary embodiments which achieve the objects, features and advantagesof the present invention. Although certain advantages and embodimentshave been described above, those skilled in the art will recognize thatsubstitutions, additions, deletions, modifications and/or other changesmay be made without departing from the spirit or scope of the invention.Accordingly, the invention is not limited by the foregoing descriptionbut is only limited by the scope of the appended claims.

1. An imaging device, comprising: a substrate; a plurality ofphotosensors arranged within said substrate to receive light; aninsulating layer formed over said plurality of photosensors; an openingprovided in said insulating layer over each photosensor through whichsaid light is received; and a light reflecting layer formed on aninternal sidewall of each opening.
 2. An imaging device according toclaim 1, wherein said opening at least partially has a shape selectedfrom the group consisting of substantially cylindrical, substantiallyconical, and curved conical.
 3. An imaging device according to claim 1,wherein the diameter of an upper portion of each opening is greater thanthe diameter at the base of each opening.
 4. An imaging device accordingto claim 1, wherein each light reflecting layer comprises a metal layer.5. An imaging device according to claim 4, wherein each light reflectinglayer comprises an aluminum layer.
 6. An imaging device according toclaim 4, wherein each light reflecting layer comprises a silver layer.7. An imaging device according to claim 6, said imaging device furthercomprising a barrier layer formed on each said sidewall beneath saidsilver layer to prevent migration of silver ions into said insulatinglayer.
 8. An imaging device according to claim 1, further comprising anetch stop layer formed over each photosensor below said insulatinglayer.
 9. An imaging device according to claim 1, further comprising anoptically transparent material disposed in each opening.
 10. An imagingdevice according to claim 9, wherein the optically transparent materialfills each opening.
 11. An imaging device according to claim 10, furthercomprising a color filter disposed over said filled openings.
 12. Animaging device according to claim 11, further comprising a lensstructure over said filter.
 13. An imaging device according to claim 1,wherein said imaging device is a CCD imaging device.
 14. An imagingdevice according to claim 1, wherein said imaging device is a CMOSimaging device.
 15. An imaging device according to claim 1, wherein saidphotosensors are selected from the group consisting of photodiodes,photogates, and photoconductors.
 16. A CMOS imaging device, comprising:an insulating layer; a CMOS array of photodiodes arranged within saidinsulating layer; a plurality of openings in said insulating layerformed over a respective photodiode, each opening having sidewalls and alight-reflecting metal layer formed on said sidewalls, the metal layerreflecting light received within its opening to its respectivephotodiode, each opening being filled with an optically transparentmaterial; a color filter layer disposed over said array of photodiodes;and a lens layer disposed over said color filter layer.
 17. An imagepixel array in an imaging device, comprising: a plurality of photoncollectors formed over a plurality of photosensors for receiving lightenergy, each photon collector having an interior space capable ofreceiving light and reflecting the light to at least one of saidphotosensors, said interior space being defined by reflecting innersurfaces of each photon collector; and a color filter formed over eachsaid photon collector.
 18. An image pixel array according to claim 17,wherein each photon collector receives and reflects light to a singlecorresponding photosensor.
 19. An image pixel array according to claim17, wherein each reflecting inner surface comprises a layer of silver.20. An image pixel array according to claim 17, wherein each reflectinginner surface comprises an aluminum layer.
 21. An image pixel arrayaccording to claim 17, wherein each photon collector reflectssubstantially all incident light internally.
 22. An image pixel arrayaccording to claim 17, wherein each photon collector has a shapeselected from the group consisting of substantially cylindrical,substantially conical, curved conical, and combinations thereof.
 23. Animage pixel array according to claim 17, wherein a diameter of an upperportion of each photon collector is greater than a diameter at the baseof each respective photon collector.
 24. A method of fabricating animaging device, said method comprising: forming a plurality ofphotosensors on a wafer; forming a structure with sidewalls in aninsulating layer on said wafer provided over said photosensors; anddepositing a reflecting material on said sidewalls of each structure.25. A method according to claim 24, wherein said reflecting material isa metal.
 26. A method according to claim 25, wherein said reflectingmaterial is aluminum.
 27. A method according to claim 25, wherein saidreflecting material is silver.
 28. A method according to claim 27, themethod further including forming a barrier layer between said silver andsaid insulating layer.
 29. A method according to claim 24, wherein thediameter of an upper portion of each structure is greater than thediameter at the base of each said structure.
 30. A method according toclaim 24, wherein said sidewalls of each structure are selected from thegroup consisting of substantially perpendicular, substantially diagonal,curved surfaces, and combinations thereof.
 31. A method according toclaim 24, further comprising depositing said reflecting layer through acollimated structure.
 32. A method according to claim 31, wherein saidwafer is held at an angle to the collimation angle of said structure.33. A method according to claim 32, wherein the angle is selected suchthat reflecting material is not deposited over said photosensors.
 34. Amethod according to claim 24, further comprising providing an opticallytransparent material contained within the sidewalls of each saidstructure.
 35. A method according to claim 34, wherein the opticallytransparent material is selected from the group consisting of spun-onglass and photoresist.
 36. A method of fabricating an imaging device,said method comprising: forming a photodiode beneath an insulatinglayer; providing an etch stop layer over said photodiode; etching anopening in the semiconductor substrate above the photodiode to said etchstop layer; depositing a light-reflecting metal on sidewalls of saidopening with a collimated source of said light-reflecting metal; andfilling said opening with a optically transparent material.
 37. A methodof obtaining an image using an imaging device, said method comprising:targeting an object with an imaging device having a lens structure;passing light reflected from said object as focused light through saidlens structure and into a plurality of photon collectors formed in aninsulating layer; reflecting at least some of the focused light off ofreflecting layers provided on sidewalls of said photon collectors; andsensing said focused light passed into said plurality of photoncollectors with an array of photodiodes arranged to respectivelycorrespond with said photon collectors.
 38. An imager system,comprising: (i) a processor; and (ii) an imaging device coupled to saidprocessor, said imaging device comprising: an insulating layer; aplurality of photodiodes beneath said insulating layer; a plurality ofphoton collectors, each photon collector being formed in said insulatinglayer over a respective photodiode, each photon collector arranged toreceive light and reflect the light within an interior space of thephoton collector to said photodiode; and a reflecting layer deposited onsaid inner surfaces of each photon collector.
 39. An imager systemaccording to claim 38, further comprising a lens structure disposed oversaid photon collectors.
 40. An imager system according to claim 39,further comprising an etch stop layer provided over said photodiodes.41. An imager system according to claim 38, further comprising anoptically transparent material in said interior spaces of said photoncollectors.